Silk, the most elegant fiver, was discovered in China by Empress Si Ling Chi when she was sitting beneath a mulberry tree in the palace garden enjoying a cup of tea. Suddenly, a cocoon, which had been attached to the tree, fell into the Empress’ tea cup. Attempting to remove it, she was fascinated to discover a very fine thread started to unravel.
The filaments of silk that form the cocoon are triangular in shape and are composed of two fibers of fibroin and a supporting matrix of sericin. Sericin makes up 20% to 30% of the filaments and acts as a support to glue the cocoon together and also as a lubricant during spinning.
Fibroin has an average diameter of about 10 to 18 microns and is a protein that abounds in non-polar amino acids such as glycine and alanine. Whereas sericin is a protein that is water-soluble and composed of amino acids such as serine and aspartic acid. The sericin that envelopes the filaments is highly tenacious and requires high temperature alkaline processing to remove it completely in order to obtain raw silk. The tenacity and gum-like quality of sericin itself makes if a good candidate for biomedical joining and sealing applications.
Physical properties
Silk fibers have a triangular cross section with rounded corners. This allows light to hit at many different angles, so silk is a bright fiber and has a natural shine. It has a smooth, soft texture that is not slippery, unlike many synthetic fibers. Its denier is 4.5 g/d when dry and 2.8 to 4.0 g/d when wet. Silk is one of the strongest natural fibers but loses up to 20% of its strength when wet. It has a good moisture regain of 11%. Its elasticity is moderate to poor. if elongated even a small amount it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty. Silk is a poor conductor of electricity and thus susceptible to static cling.
Unwashed silk chiffon may shrink up to 8% due to a relaxation of the fiber macrostructure. So silk shoulder either be pre-washed prior to garment construction, or dry cleaner. Dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage or shrinkage due to molecular-lever deformation.
Silk is sensitive to higher temperatures. It should be, therefore, subjected to ironing at moderate temperature. Silk is sensitive to light than any other natural fibers. Yellowing of the fiber is generally accompanied by photo-degradation that is mainly occurred due to action of UV radiation of light.
Chemical properties
Action of Acids: Hot concentrated acids readily decomposes silk. The degree of hydrolysis is much greater with acid than with alkali and is pH dependent. Weak (5% solution) and cold hydrofluoric acid does not have any harmful action on silk. Dilute organic acids show little effect on silk at room temperature.
Action of Alkali: Dilute alkali temperature does not cause any considerable chemical and structural damage of silk, but the luster of the fiver may be decreased depending on the exposure time. Caustic soda, when it is hot and strong, dissolves the silk fiber.
Action of oxidizing and reducing agent: Silk fibroin is not severely affected by hydrogen peroxide solution. The action of chorine solution on the silk fibroin is more harmful than does the solution of hypochlorite. The reducing agents that are commonly found in use in textile processing such as hyposulphite, sulfurous acids and their salts do not exercise any destructive action on the silk fiber.
Uses of Silk
Silk’s good absorbency makes it comfortable to wear in warm weather and while active. Its low conductivity keeps warm air close to the skin during cold weather. It is often used for clothing such as shirts, blouses, formal dresses, high fashion clothes, negligees, pajamas, robes, skirt suits, sun dresses and underwear.
Silk’s elegant, soft luster and beautiful drape makes it perfect for many furnishing applications. It is used for upholstery, wall coverings, window treatments (if blended with another fiber), rugs, bedding and wall handing. Silk is also used for parachutes, bicycle tires, comforter filling and artillery gunpowder bags.
Early bulletproof vests were made from silk in the era of black powder weapons until roughly world war I. A special manufacturing process makes it suitable as non-absorbable surgical sutures. Chins doctors have used it to make prosthetic arteries. Silk cloth is also used as a material to write on.
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